Design Help Needed

[mag998]

Hi all,

I had designed my system to use other charge controllers but when I found the Midnite Classic, I realized things would have to change quickly! I have now ordered the Classic 150 based on the recommendation from the sizing tool (see attachment) and the fact that I can still add two more strings with two panels each without having to add another controller. But now I need some help from you folks. I have a off-grid system hooked up through two inverters and double manual transfer switches. I'm not planning to use an E-panel but I need breakers and boxes for those. My panel to charge controller run is ~75'. I would be very grateful if you can take a look at some questions I have around combining and cable runs:

1. Should I place a disco box with breakers outdoors close to the panels or should I put it next to the Classic 150? Or both??
2. If I put it outside, how much voltage/amps can I go to safely in my 6 AWG cables?
3. What breakers will I need, how many and how many amps? Outside or inside?
4. Can I use the Big baby box with breaker(s) between the Classic 150 and the batteries?
5. Which SPD should I use, and where do I install it?
6. Do I need separate Battery Disconnect Switches between batteries and inverters? Any recommendations? I have a 300A fuse on 2\0 AWG cable there already.
7. My Samlex inverter manuals state that they have neutral tied to ground. Do I need to separate those? I read somewhere this should only be tied in the panel.

Keep in mind I will eventually expand to a 2.5kW system with ten panels in total. Thank you for taking the time to help me with these. Please let me know if you have any questions!

Cheers,
-magn

[laszlo]

Hi!

There are several questions but the disco should be placed on the outside wall where the PV conductors  enter the building or structure. There is label "PV disconnect" that you can purchase online and is required by code to be attached to teh disconnect device.

#6 conductor ampacity depends the insulator temperature rating and  temperature rating of the breaker terminals. The key words are, it depends.

[Vic]

Just quickly,

ASSUME that you are in NA,   probably in the USA -- Codes differ depending on where one is located.  Codes are good to follow,  generally.  Your system may or may not be subject to Inspection.  If the system is subject to inspection,  the AHJ will have the final word on what is required and just where the important things need to be.  Many off-grid systems do not require inspection,  but it is good to know.

Devices like fuses and breakers generally do not protect equipment,  but rather protect wiring and cables (from fire).  Circuit breakers are also often convenient disconnecting devices.

So,  sizing the conductors depends upon the maximum current flowing,  the temp rating of the insulation (as Laszlo mentioned),  and if the cables are in conduit,  the number of cables carrying current that are in that conduit,   and often some safety factor needs to be applied.  AND,  as you know often one needs to use larger conductors than the minimum size based on the current flowing because of voltage drops in longer lines (especially on lower voltage DC circuits).

The Baby,  Big Baby,  and Quad boxes should be fine for many of the breakers.  A real PV Combiner box has some advantages.  You will need breakers or fuses in the combiner as believe you are using three strings of PVs now.  The MNEPV breakers will be fine for the Combiner.  You can use the Maximum Fuse rating stated on the label of the PV to size that breaker.

Good that you are familiar with e-Panels.  They save time,  space,  and some clutter when used.  You might want to reconsider an e-panel,  as every breaker that you will be using inside the power room should be securely mounted,   enclosed,   and generally interconnected with conduit.

A circuit breaker between the battery and each Inverter can be very convenient for testing and servicing the system.  Fuses CAN do the job of protection,   but  blowing fuses can be expensive,  and is inconvenient.

Just how many SPDs and just where they should go really depends upon the exact layout of the system,  and just where things are located.

More later.   Good questions.   Vic

[mag998]

Howdy and thanks both of you for your answers.

I am located in Texas. I'm planning to put the disco box with 30A breakers and SPD on the inside of the garage wall where the PV feed enters, close to the Classic 150, then go from the Classic to the Big Baby with a 63A (later up to 80A or 100A) circuit breaker going to the battery banks. I have also obtained a pair of marine style 450A disconnect switches for the inverter cables.

In the future as I add more PV arrayss I may add a disco switch with breakers out at the panels to bump the voltage up to 48V.

How would the number of cables in the conduit affect the max amperage? Do you know of a formula or calculator for this?

Cheers,
-mag

[Vic]

Hi mag,

Thanks for the added info.

Here is an Ampacity Table --  allowable current in conductors.   There are correction factors for ambient temperatures at the bottom:

http://www.houwire.com/products/technical/article310_16.html

If your system will be inspected,  there are additional temp correction factors for exposed conduit on roof-tops in the later Code.

Generally,  the Ampacity tables are for three or fewer current-carrying conductors in a cable, or conduit.  With 4-6 current carrying conductors,  the table data is reduced to 80% of the listed value.  7-9 conductors the factor is 70%.

As you probably know,  there is also a conduit fill,  which indicates the maximum number of a certain size wire of a certain construction (insulation thickness etc) that are also listed in Code tables.

As laszlo mentioned before,  there is also an additional factor regarding the maximum amount of current that a certain size wire can handle.  This is the temperature rating of the terminal to which it is connected.  90 degree C wire connecting to a breaker with a terminal rating of 75 C must be de-rated to the max current allowed for 75 C.

I am not a Code jock,  and this level of detail here may not be of interest to a great number of members.  The Code contains a lot of detail.  Most of it is well thought out.  Some of us have begun to wonder if it has gone a bit too far vs what are the costs of trying to comply,   BUT,  if one's system will be inspected,  it needs to be done to Code,  and it is the inspector who will tell you what is needed,  even if it is not specifically in the Code. The primary purpose of the Electrical Code is for safety.  Doing systems to Code,  even if not required is usually a good idea (within reason).  Just my opinions.   Good Luck,  Vic

[mag998]

Hi and thanks..!!

yes, I actually used this calculator that is based on that table:
http://www.nooutage.com/vdrop.htm

Cheers,
-Magnus

[laszlo]

Mag,

Looks like you are in good company now. Thanks for the additional detail. As Vic says the ampacity calculations can get a bit esoteric, and a lot of it  depends on local factors.

Here are some other point to mull on:

-the Classic will limit the current based on amps out  setting,  so I'd make sure if you have breaker size of 63amps then you also limit Classic output to 63 amps (and later adjust it) For 63 amps you have to use #4 wire os that section from you combiner into the DC breaker and then into the CLassic needs to be #4.

-the use of the disco is often in scenarios like fire where you or the firemen want quickly to be able to shut off all PV inputs from outside of the building instead of having to get inside a building. It's a good practice and if you wnat somebody to buy your place later, *they* would want safety features like that. And 2-5 years from now PV will be commonplace,   and people will just be looking at PV as an investment or as a safety liability. So what you can add in safety now is going to be valuable later.

[mag998]

Thanks Laszlo. I agree, it makes sense..! I hope to have this hooked up this weekend, will let you know how it goes.

Cheers,
-mag

[mag998]

Hi all,

my system has been running almost flawlessly for several months now. Really the only thing I have noticed is that the Classic 150 has lost the kWh log for the day a few times, for less than 5 days in about 5 months. I have added two more 250W panels so right now I have 2kW max generating power and I have increased my battery bank size to 1025Ah (24V). As I mentioned earlier, the Midnite breaker between the Classic and the battery bank is 63A. I have limited the output of the Classic to 63A. However, yesterday I noticed that the local app went yellow, meaning the Classic hit this limit of 63A. I figured I should test the breaker and went out and increased the limit to 70A. The Classic went as high as 68.2A temporarily  but the breaker never opened. Is this a faulty breaker? Would the breaker open immediately or is there a delay? Also, I am ready to swap to a new 100A breaker so I can take full advantage of the Classic's max charging amperage, but now I'm concerned as I won't be able to test if that one works as designed... finally, will #4 cable still be okay for the 92A max charging amps? The cable is about 4' in total, including breaker.

Cheers,
-mag

[Vic]

Hi mag,

Thanks for the update.  Great that your system is operating well for you.

The MidNite breakers are rated for continuous currents at the rating of the breaker,  WITHOUT TRIPPING.

You can see the curves at the bottom of this CBI (the breaker manufacturer) document:
http://www.midnitesolar.com/pdfs/Q-Series(1).pdf

EDIT:  I do not know exactly which curve has been specified by MidNite for their MNEPV breakers,  but you can see that essentially any of the curves will allow a reasonable amount of time at significant over-currents without tripping - end edit.

The MN CBI QY Breakers are listed for 150 VDC for the MNEPV models.

As a technicality,  these breakers are listed for rated current when in MN listed enclosures.

So,  the breaker should never trip when running continuously at the max rated current of the breaker.

There should really be no need to test breakers.  Am certain that they are 100% production-line tested for function.

#4 AWG THHN building wire is rated for 95 A for three or fewer current-carrying conductors in conduit or raceway.

This should be OK in when using customary installation practices.  There could be a Code issue here or there depending on exactly how your installation is made.  I am not a Code Wizard.

You should not need to test your breakers.  And,  IMHO,  would rather NOT subject breakers to tripping DC breakers at or near rated currents.  These breakers are rated for thousands of trips at rated current,  but it is just my personal feeling that it is better to not do it.

Just be certain to keep the connections on the breaker,  the Classic terminals,  and on the Battery Bus-bar tight,  and you should be fine.   Just my opinions.   Vic

[zoneblue]

Ditto, ditto on the breakers. Also remember that the 63 amp din breakers are in positive to the bank. So tripping it from excess controller amps will effectively reverse trip the breaker, which is generally considered a 'let the smoke out' kind of thing.

However ive heard Robin G talk about how they tested these, and he reckoned hte big failures were at high end of voltages. What would happen at 24V is unclear.

[mag998]

Vic, thank you so much for your response, and in particular for the diagrams. These are marked with the "U2" curve. Looks like I was just a few seconds away from actually tripping today during my tests then .

zoneblue, good point. I have wondered how that would work as the plus is on the battery side. What is the best way to do this safely then? Put two breakers in series, in opposite direction???

Cheers,
-mag

[zoneblue]

Those polarised breakers are in the process of being phased out, ie wont be allowed for new installs next year or so, at least for compliant stuff any ways.

Im using the din ones, and in the same boat, however i actually originally specified and bought was the 100A panel mount breaker, but in the build wanted to keep it all din for ease of mounting. Midnite do now sell a din rail 100amp breaker, it fits two poles wide. But as to when the breaker guys gear up to make these new non polarised dc din rail breakers is any bodys guess. Its not exactly a big market.

Actually i do posess a couple of din 63amp 500V dc non polarised breaker, Noarc brand. No idea what thier interupt rating is like. They were 'loaned' to me.

[mag998]

Hi again and thanks.

I just tested the 100A breaker, it opened both ways after about 10-20 seconds at 105-120% amperage, which looks about right according to the curve. Very happy it worked, will install now.

Cheers,
-mag

[mag998]

Hi all,

since a couple of days ago my Midnite is throwing a Ground Fault. It starts beeping and stops charging. I have isolated the faults to one of the solar array. When I disconnect both the positive and negative cables on that array, the charger stops beeping.  Does anybody have any experience from this happening? Any suggestions? I have check the connections and everything seems fine.

Cheers,
-mag